Plasmonic imaging of the layer-dependent electrocatalytic activity of two-dimensional catalysts

Studying the localized electrocatalytic activity of heterogeneous electrocatalysts is crucial for understanding electrocatalytic reactions and further improving their performance. However, correlating the electrocatalytic activity with the microscopic structure of two-dimensional (2D) electrocatalysts remains a great challenge due to the lack of in situ imaging techniques and methods of tuning structures with atomic precision. Here, we present a general method of probing the layer-dependent electrocatalytic activity of 2D materials in situ using a plasmonic imaging technique. Unlike the existing methods, this approach was used to visualize the surface charge density and electrocatalytic activity of single 2D MoS2 nanosheets, enabling the correlation of layer-dependent electrocatalytic activity with the surface charge density of single MoS2 nanosheets. This work provides insights into the electrocatalytic mechanisms of 2D transition metal dichalcogenides, and our approach can serve as a promising platform for investigating electrocatalytic reactions at the heterogeneous interface, thus guiding the rational design of high-performance electrocatalysts.


Plasmonic response to charge density change of MoS2
In the surface plasmon resonance (SPR) imaging setup, a beam of p-polarized monochromatic light passes through a high numerical aperture objective to illuminate the gold-coated coverslip.
When the incident angle is modulated, a sharp decrease in reflectivity can be monitored with a camera. The angle at the reflectivity minimum is referred to as the resonance angle. The resonance angle (θSPR) is given by Here we sought to simulate the SPR reflectivity profile of monolayer MoS2 at different applied potentials for the optical multilayer interface 6 . The Transfer Matrix method is based on the continuity conditions for the electric field across boundaries from one medium to the next, according to Maxwell's equations. In our experimental system, the reflections of light in the multiple interfaces are partially transmitted and reflected. The overall reflection of a layer structure is the sum of an infinite number of reflections ( Supplementary Fig. 2a). The refractive index of MoS2 under different potential was acquired from the literature 7 . The increased electron density of MoS2 decreases its refractive index. In Supplementary Fig. 2b and c, the application of a negative potential on the MoS2 shifts the θSPR towards lower incidence angles. A convenient way to determine the potential-dependent θSPR is to fix the incident angle near the resonant angle (71.5•, dashed line in Supplementary Fig. 2c) and measure the plasmonic intensity change in the reflected light. As a result, the plasmonic intensity of monolayer MoS2 decreases with the increase in electron density.

Image processing method
In the charging experiments, the electrochemical recording was synchronized with the camera by adding a timeline into the image sequences using a data acquisition card (DAQ, USB-6250, National Instruments). We subtracted the first image of each sequence (Supplementary Fig. 3a-b).
With the timestamps, we selected one of the 2 points in the sine wave of the voltage timeline, as shown in Supplementary Fig. 3c Fig. 3j), the optical response was finally transferred into the localized current density.
All analyses above were carried out by custom-written MATLAB code, which is available upon request.

Equivalent circuits for the charging experiment
Considering the absence of redox species in the charging experiment, the electrochemical system was dominated by the interfacial capacitances and resistance in series. As shown in the equivalent circuit ( Supplementary Fig. 4a), the interfacial capacitance in the monolayer MoS2 area consists of electrical double layer capacitance (CEDL) and quantum capacitance (CQ) in series: The CEDL is mainly dependent on the electrolyte concentration and ion species. In 0.26 M phosphate buffer, the CEDL was approximately 25 μF cm -2 8 . The CQ of monolayer MoS2 was calculated to be 5 μF cm -2 by the following equation 9 : where 2 is the band-edge DOS, and EG is 1.9 eV for the monolayer MoS2. Vg is the gate potential, -0.2 V. More detailed calculation for quantum capacitance was provided in Supplementary Note

14.
Traditional electrical methods measure the average signal of the whole working electrode. In The surface charge density of single MoS2 nanosheet is related to local interfacial capacitance per unit area (c), Δq = cΔV. Accordingly, we need to obtain the interfacial capacitance of the MoS2 region instead of the total capacitance of the whole working electrode. Previous literatures have also adopted the series-connected capacitors of CQ and CEDL to represent the interfacial capacitance of the semiconductor-electrolyte interface ( 1 = 1 + 1 ) 13,14 . Therefore, it is reasonable to use the CQ for the final capacitance of the MoS2-electrolyte interface, regardless of the right loop describing the Au/SAM/electrolyte region.

Cyclic voltammetry of the bare Au and 1-octadecanethiol-modified Au electrodes
The Au electrode was passivated by a self-assembled monolayer (SAM) of 1-octadecanethiol molecules to hinder electron transfer from the gold electrode to the redox molecules. To demonstrate the effectiveness of blocking, we simultaneously studied the redox reaction of the Ru(NH3)6 3+ complex with conventional CV and plasmonic imaging methods. As shown in Supplementary Fig. 6a,

Quantification of the noise level
We quantified the noise level of the plasmonic response for the electrochemical redox reaction.
Along with the potential modulation, the plasmonic response of monolayer  Fig. 9a, orange). We observed a one-to-one correspondence between the measured plasmonic intensity change and the simulated plasmonic intensity change determined according to Eq. 2.
To validate the relationship between the concentration ratio of [Ru(NH3)6] 3+/2+ and the measured plasmonic intensity change, we converted the measured current to the plasmonic intensity, or vice versa 17 . Supplementary Fig. 9b (the orange curve) shows the semi-integral of the conventional cyclic voltammogram acquired on an electrochemical workstation, which was in accord with the measured plasmonic intensity change (the blue curve). Additionally, the currentpotential curve converted from the measured plasmonic intensity coincided with the conventional CV measured with an electrochemical workstation ( Supplementary Fig. 9c). These two curves were overlayed accurately, indicating that the plasmonic intensity change and the conventional CV could be converted to each other. When the redox reaction was performed on the 1octadecanethiol-modified gold electrode, the plasmonic intensity change in 10 mM [Ru(NH3)6]Cl3 presented a similar plasmonic intensity shift ( Supplementary Fig. 9d). This result indicates that an alkanethiol layer could block the electrochemical reaction of the gold substrate without affecting the redox reaction of monolayer MoS2.

Conversion of the plasmonic intensity of single MoS2 nanosheets to electrochemical current
The electrochemical current of monolayer MoS2 was determined with the formula described previously 16,20 . The quantitative relationship between the plasmonic signal and the electrochemical current of monolayer MoS2 is given by According to the calculation process described above, the calculated i( ) is the current derived from the layered MoS2. Considering that the thiols could block the signals of the gold substrate, the measured ( ) does not contain the double layer charging current. Therefore, the measured ( ) merely reflects faradaic processes related to redox reactions.

Catalytic kinetic analysis of MoS2 with different thicknesses
To understand the relative rate of electron transfer at single MoS2 nanosheets with different thicknesses, we extracted the half-wave potentials (E1/2) of MoS2 catalysing redox couples for kinetic comparison.

Mapping the surface charge density of single MoS2 nanosheets with various thicknesses
To unravel the layer-dependent electrocatalytic activity of the MoS2 nanosheets, we mapped the surface charge density of single MoS2 nanosheets with various thicknesses.
The calibration factor of the surface charge density and the plasmonic intensity is depended on the interfacial capacitances which is the layer-dependence CQ, mainly. We firstly calculated their electron density-of-states (DOS) based on density functional theory (DFT) for further CQ calculation. All DOS calculations of MoS2 on Au (111) were carried out with DFT calculations using the CASTEP code, which has been widely applied to metal surfaces. All-electron calculations were employed using the generalized gradient approximation (GGA) and the Perdew, where the φ is the applied potential. Supplementary Fig. 13b shows the determined CQ of MoS2 with different number of layers.
Considering that the PBE functional has some limitations for the non-metallic systems, we also performed the DFT calculations using the HSE06 functional with the same conventional cell structure used above. Previous studies have proved that the results from PBE and HSE06 functionals can be mutually verified by calculating a variety of semiconductor materials 21,22,23,24 .
The HSE06 hybrid functional can perform better in band gap calculations for non-metal system.
As shown in Supplementary Fig. 13a Fig. 13b and d).
The calibration factor for the conversion of plasmonic intensity to surface charge density varies with the number of layers. In manuscript Fig. 4g, we used the as-obtained layer-dependent CQ values to calculate the calibration factor. The cited reference also used the DOS of MoS2 to calculate CQ, and our results were similar to that in the reference 9 .

Au-MoS2 Schottky contacts
In electrocatalytic process, the charge injection from the current collector to the catalyst and charge transport across the catalyst to the active sites determine the overall charge transport efficiency and thereby affect electrocatalytic activities. For semiconducting catalyst like MoS2, a Schottky barrier exists at the metal-semiconductor interface to prevent efficient charge injection 26 . The Schottky barrier height is dictated by the energy difference between the work function of the metal and the conduction band edge of MoS2. As shown in Supplementary Fig. 13, we depicted a band diagram at the Au-MoS2 interface. As the layer number increases, upshift of conduction band edge in thicker MoS2 decreases Schottky barrier height for electron injection. Therefore, the thicker MoS2 has lower Schottky barrier height than the thinner one, leading to better electron transport ability in thicker MoS2 at the Au-MoS2 interface. However, we observed a contrary trend that the surface charge density of MoS2 decreased quickly as its thickness increased in our work. That means, the electron transport across the interlayer of MoS2 dominates the overall charge transport efficiency, rather than the charge transfer between MoS2 and Au contact. Furthermore, the plasmonic imaging technique is sensitive to surface charge density, and the refractive index of

Imaging the hydrogen evolution reaction (HER) at single MoS2 nanosheets.
We measured the electrocatalytic hydrogen evolution reaction (HER) on single MoS2 nanosheets by sweeping the electrode potential between 0 and -0.8 V (vs. Ag/AgCl) in 0.25 M H2SO4, and recorded the plasmonic images over time. The generated hydrogen molecule leads to the decrease in the local refractive index around MoS2, which is reflected in the change of plasmonic image contrast. Supplementary Fig. 16a-b displays snapshots of differential plasmonic images at different potentials, where the image contrast decreases as the potential decreases, and achieves a maximum at the lowest potential. We selected a ROI around the MoS2 and obtained the plasmonic image intensity curve in Supplementary Fig. 16c. Supplementary Fig. 16d

Layer-dependent electrocatalytic activity of other two-dimensional materials
Our imaging method is facile and can be applied to probe the electrocatalytic activity of other twodimensional (2D) materials. We also imaged the electrocatalytic activity of three other kinds of 2D materials with the quantitative method in Supplementary Note 11, including graphene, WS2 and MoSe2, which exhibit similar layer-dependent electrocatalytic behaviours. Supplementary Fig.   17a-c display the cyclic voltammograms of graphene, WS2 and MoSe2 with various thicknesses determined by our electrochemical imaging technique. All of the materials exhibited faster electron transfer kinetics with decreased layer numbers due to the decreased interlayer electron transfer ability as the number of layers increases. Similar to MoS2, the half-wave potentials of graphene and WS2 shifted towards more positive potentials with decreasing the number of layers ( Supplementary Fig. 17d). Meanwhile, CV curves of MoSe2 did not show significant redox peaks in the potential range. The electron transfer kinetic of MoSe2 is much slower than that of MoS2 and WS2 under the same conditions. In general, electron transfer between interlayers in these vdW materials occurs via electron tunneling, and the interlayer conductivity can decrease significantly as its thickness increases 27,28,29 . These results suggest that interlayer electron transport plays a considerable role in the electrocatalysis of 2D materials.
Given the layer-dependent electrochemical activity of these 2D nanomaterials, it is noteworthy that only the materials with the same layer number can compare their electrocatalytic and electrochemical properties. We further compared the electrochemical response of different 2D nanomaterials with similar layer numbers (~ 8 layers). As shown in Supplementary Fig. 17e, the maximum reduction current density of graphene is larger than the other three nanomaterials, indicating a more facile electron transfer kinetic of graphene under the same number of layers. This is because graphene is a semi-metal with a high electrical conductivity and the other three TMDs are semiconducting catalysts 30,31 . However, the high electrical conductivity of a catalyst does not promise a high catalytic performance since graphene is a catalytic inert material. This is because the adsorption/desorption kinetics of the reactants on the catalyst active sites is another important factor. They both contribute to the catalytic performance of a catalyst.